Clear Spring finished it off in the bottom of the inning as Garrett Embly led off with a single to set up the rally. Jaret Lazich came to the plate two outs later and blasted his game-winning shot on a 3-0 pitch.
"It was one of the greatest games we played in all season," said Northwest Washington manager Dan Baker.
Clear Spring advances to play today at 2 p.m. against the South Maryland-Denton winner, which was played late Saturday.
Northwest Washington took a 3-0 lead in the first inning on a two-run double by George Anninos and a run-scoring single by Ben Swartz.
Clear Spring roared back with a run in the first and six in the second to take a 7-3 lead.
Clear Spring scored three runs on errors, two more on wild pitches and the last on Josh Colliflower's RBI single in the second inning.
Northwest Washington scored three times in the third on a two-run single by Anninos and a John Spaller grounder to cut the deficit to 7-6.
Zack Greenlee's double in the third drove in two runs to start a four-run rally. Colliflower added a sacrifice fly and Jaret Lazich scored on a fielding error as Clear Spring opened up an 11-6 lead.
Northwest Washington started its comeback in the fourth inning with three runs as Anninos and Ben Swartz each had RBIs. A Clear Spring error provided the third run. Northwest Washington moved to within 11-10 in the fifth when Peter Kaninsky was hit by a pitch with the bases loaded.
Northwest Washington tied the game when Beckett Norris drew his bases-loaded walk.
In the tournament opener, West Salisbury defeated Brunswick 7-3 to advance in the winners' bracket.
Cade Hurley's grand slam led West Salisbury. Hurley was also the winning pitcher for West Salisbury while Aaron Fitzwater took the loss for Brunswick.
Seth White went 2-for-3 with an RBI to lead Hughesville to a 6-5 win over Elkton in the tournament's second game. Tyler Robinson led Elkton with a two-run home run in the first inning.